RESEARCH ARTICLE
O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
1
Understanding the Precipitated Calcium Carbonate (PCC)
Production Mechanism and Its Characteristics in the
Liquid–Gas System Using Milk of Lime (MOL) Suspension
a
b
a
Onimisi A. Jimoh , Tunmise A. Otitoju , Hashim Hussin ,
Kamar Shah Ariffina,* and Norlia Baharuna
a
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal,
Penang, Malaysia.
b
School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia.
Received 3 April 2016, revised 15 August 2016, accepted 5 September 2016.
ABSTRACT
This study investigates the effect of operating variables and influence of milk of lime (MOL) conditions in PCC using a modified
reactor. The variables includes: Ca(OH)2 feed concentration at 0.5 M–2.0 M and CO2 flow rates at 224.0 mL min–1 and
379.5 mL min–1, on the particle morphology and size in the gas–liquid route precipitation. The particle morphology and texture as
well as the chemical content were sufficiently authenticated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and X-ray fluorescence (XRF). Experimental data show that lower concentration (<1.0 M) favoured the formation of rhombohedra calcite crystals with the particle size below 100 nm. However, increase in
concentration and gas flow rate yielded a coarser crystal particles. Two polymorphs were produced at 1 M reactant, i.e. rhombohedra
calcite with CO2 flow rate of 224 mL min–1 and prismatic calcite at 380 mL min–1. Molarities higher than 1 M yielded a coarser
prismatic crystals, and also has a tendency to crystallize into scalenohedron species especially with higher reactant concentration.
KEYWORDS
Milk of lime, precipitated calcium carbonate, morphology, particle size, carbonation.
1. Introduction
Precipitated calcium carbonate (PCC) is a filler used in many
applications, like papers, plastics, rubbers, paints, drugs and so
on. Its high purity, well-ordered particle size and morphology
makes it the white filler of choice.1 In paint production, PCC
has established itself as a primary extender due to its unique
properties such as its low basic colour, high weather resistance,
relative abrasiveness, low electrolyte content, non-toxicity and
the pH stabilizing effect. Different kinds of crystal are suitable
for a particular application and only the right PCC can boost
the quality of the end products. Precipitated calcium carbonate
produced with a prismatic and rhombohedral-shape has maximum light dispersion at 0.4 to 0.5 µm sized particles while a
scalenohedral-shaped precipitated calcium carbonate has maximum light dispersion of 0.9 to 1.5 µm particles.2 PCC of
nanometer sized with rhombohedral morphology are highly
effective for use as coating in a paper making.3
There are several approaches in the prior art to synthesize PCC
having definite property like high purity, most of which are
conversely focussing on this single property only. Although the
processes do not allow full control of other properties such as
crystal morphology, particle sizes, etc.4
The advantage of PCC lies in the prospect of tailor making the
products with definite particle morphology, particle size and
distribution as well as specific surface area (BET). PCC is a
refined natural form of limestone using techniques. The purity
can be increased to 99.9 % or 99.999 %, and grain size can be
controlled in a range from submicron to more than 10 micron.5
Calcium carbonate occurs in different crystalline polymorphs at
* To whom correspondence should be addressed. E-mail: [email protected]
ambient pressure. There are anhydrous phases of aragonite,
vaterite, calcite and hydrated phases of monohydrocalcite as
well as hexahydrocalcite.6 The anhydrous CaCO3 can be classified as rhombic calcite, needle-like aragonite or spherical
vaterite and among them calcite is known to be the most stable
phase under ambient atmospheric conditions.7 The formation of
any of these three polymorphs is strictly dependent on some
parameters such as the temperature, supersaturation and pH of
reaction solution.8 The control of calcium carbonate polymorphism
is an intricate interplay amid thermodynamic and kinetic factors.9
The work by Ciullo10 shows that high purity PCC (>95 %) can be
produced using pseudo-catalytic lixiviant to selectively extract
calcium from slag material before being dissolved as PCC. Also
Adams11 showed that high purity PCC can be produced from
steel slag using ammonium salts to selectively extract calcium
from slags. They reported that the smallest solid to liquid ratio of
5 g L–1 resulted in maximum calcium extraction efficiency (73 %)
while using 100 g L–1 produced the lowest extraction efficiency of
6 %. Liu et al.12 tested the performance of two organic acid
(succinic and acetic acid) for the possible extraction of calcium
from steel slag for PCC production. They reported that the
carbonation of succinic acid leachate did not result in the production of PCC, whereas, the carbonation of acetic acid leachate
resulted in the synthesis of PCC.
A study by Suwanthai et al.13 shows that high purity PCC
(mainly calcite) can be synthesized from gypsum waste by using
an acid gas (H2S) to improve the aqueous dissolution of the
poorly soluble CaS14, produced high purity PCC (mainly amorphous) from medium and low grade limestone using strongly
acidic cation exchange resin. This improvement was due to
ISSN 0379-4350 Online / ©2017 South African Chemical Institute / http://saci.co.za/journal
DOI: http://dx.doi.org/10.17159/0379-4350/2017/v70a1
RESEARCH ARTICLE
O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
the reaction of HCO3– in aqueous solution during the slaking
reaction process. Valuable information from their work shows
that they were able to eliminate the influence of impurities
during the carbonation reaction. Furthermore, Chen et al.15
studied a reaction precipitation of calcium carbonate by bubbling
CO2 gas through a double-tube gas injection nozzle into milk of
lime (MOL) solution in an agitated closed batch reactor. In their
study, they observed that particle morphology and nucleation
were closely related to the reaction pH of solution.
The recarbonation process is the most common technique of
producing PCC. It is a complex process and involves simultaneous
dissolution of Ca(OH)2 and CO2 as well as the crystallization of
CaCO3. Carbonation is generally carried out in a series of reactors
under closely controlled operating conditions to produce the
required PCC morphology and particle size distribution (PSD).
Luo et al.16 studied the synthesis of vaterite PCC via carbonation
route under controlled pH condition. They reported that buffer
solution highly influence the polymorphic phase of PCC while
CO2 concentration and rate of gas flow have no significant influence. Furthermore, high purity spherical vaterite particles can be
synthesized under controlled reaction conditions.
This work investigates the transition effects of reactant concentration and gas flow rate on PCC morphology and crystallite
size using gas–liquid route via a modified batch-wise, bubblecolumn type reactor.
2. Materials and Experimental Methodology
In this research, limestone used for the experiments was
calcined in a muffle furnace at an optimum disassociation
temperature and resident time. The raw materials used in this
research are high purity limestones, which are generally free
from any major contaminants (purity >98 %) for ‘milk of lime’
with less than 2 % impurities made of mostly silica (SiO2). However, the impurity does not present any significant discoloration
in the final PCC.
Chunks of limestone were crushed into a manageable size
range of 10–20 mm diameter before calcined in a muffle furnace
at an optimum disassociation temperature and resident time.
High reactive and low decrepitating quicklime (CaO) was
attained at an optimum calcining temperature of 1100 °C at
90 min. The resulted quicklime is then hydrolyzed into distilled
water to produce creamy hydrated-lime. In the slaking process,
lumps of quicklime disintegrated to produce creamy white
smooth paste of hydrated-lime Ca(OH)2 suspension, and is subsequently diluted in excess distilled water to produce different
‘milk of lime’ molarities. The resulting ‘milk of lime’ suspension
is then screened to remove any coarse grits and impurities to less
than 105 µm. Presence of trace amount of aluminium-silicate
bearing minerals such as fine clay particles, carbonaceous
matters, over burned or residual calcium carbonate generally
has insignificant effect on the slaking process. The resulted PCC
produced from various experimental designs were carried out to
characterize its crystal morphology, particle size distribution
pattern, state of agglomeration, and chemical purity. Table 1
shows the experimental designs for these experiments.
2.1. Precipitation/Recarbonation Process
These experiments were carried out in a one liter modified
batch-wise, bubble-column type reactor (Fig. 1) containing
500 mL milk of lime with a constant stirring speed of 300 rpm (the
optimum stirring speed in this experiment). The experiment
was designed to investigate and foresee the influence of reactant
concentration and gas flow rate towards resulted PCC characteristics and precipitation process mechanisms. This is especially in
2
Table 1 Experimental designs for the experiments.
Experiment
No.
Reactant concentration
/M
Gas flow rate –CO2
/mL min–1
PCC A1
PCC A2
0.2
224.0
379.5
PCC B1
PCC B2
0.56
224.0
379.5
PCC C1
PCC C2
0.8
224.0
379.5
PCC D1
PCC D2
PCC D3
1.0
224.0
379.5
PCC E1
PCC E2
1.5
224.0
379.5
PCC F1
PCC F2
PCC P8
2.0
224.0
379.5
the terms of crystal morphology and growth sizes of PCC
formed under specific operating conditions and parameters
setting. In this case, precipitation or recarbonation processes
were conducted at two predetermined CO2 gas flow rates over a
period of 60 and 90 min resident time. Reactant concentrations
were fixed between 0.2 and 2.2 M, and conducted at ambient
temperature and pressure conditions, respectively.
The pH, reaction temperature, conductivity/resistivity and
total dissolved solid (TDS) of the reactant during the stages of
recarbonation process were closely monitored (using Istek
K4000-EC) for the product characteristics and process mechanism
analysis. The resultant PCC after recarbonation were recovered
by filtering through membrane filter, washed using distilled
water and dried at 105 °C for 24 h and subsequently analyzed for
morphology, particle size distribution, and purity. Effects of the
concentration and CO2 flow rates on particle morphology and
size were investigated. The degree of recarbonation to form PCC
was determined by loss on ignition (LOI). The morphology
and particle size of PCC of each samples were examined under
scanning electron microscope (SEM). Prior to SEM examination,
these PCC powders were dispersed in methanol and ultrasonically treated to reduce particle aggregation.
3. Results and Discussion
In the carbonation process, solid Ca(OH)2 is first dissolved as
soluble Ca2+ and OH– ions, while CO2 is absorbed in water in the
form of weak H2CO3 and then converted to H+, HCO3– and CO32–
ions. Then Ca2+ combines with CO32– to form CaCO3, whilst H+
and OH– merge to form H2O. The rate of recarbonation seems
directly related to the reactant concentration, bubbling time and
CO2 gas flow rate conditions, and eventually affects the characteristics of the resulting PCC.
Ca(OH)2(aq.) + CO2(g) Þ CaCO3 + H2O
3.1. Effects on Conductivity and pH
Reactant concentration at a particular time during the course
of the reaction can be monitored by conductivity and pH.
During the course of precipitation, the pH, conductivity (Fig. 2)
and TDS values gradually decreases in tandem with the progress of recarbonation process and abruptly dropped to the lowest level at specific moments during the course of precipitation.
At low initial concentrations, the solid Ca(OH)2 is rapidly dissolved and depleted in a short period of time during the course
RESEARCH ARTICLE
O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
3
Figure 1 Modified batch-wise, bubble-column type reactor.
of precipitation to form PCC crystallite. Rapid consumption of
Ca2+ and OH– witnessed a decrease in pH of 12 to neutral (pH 7),
TDS and conductivity values.
This consequently will affect the characteristics of produced
PCC, e.g. particulate sizes and crystal morphology. Dissolution
of Ca(OH)2 and CO2 increases the concentration of ionic charges
dissolved species, i.e. Ca2+, OH–, HCO3–, CO32–, CaOH+ and
CaHCO3+ during recarbonation. Progress in recarbonation,
gradually generated CaCO3 in the form of solids particles (less
dissolved species), hence reduces the electrokinetic ability as
well as resulted to decrease in conductivity ability. This indicates
that a complete PCC precipitation process was attained at a
point where there were no more excess Ca2+ available. Higher
reactant concentrations need a longer bubbling time, and this
can be compensated by increasing the gas flow rate. Excess gas
supply only resulted in dissolved CO2 to form a weak HCO3–,
and normally show a slight rebound in conductivity values
(Fig. 2).
3.2. Effect on Resistivity, Total Dissolved Solid (TDS) and
Temperature
Figure 3 shows a sudden and brief peak-up in resistivity
Figure 2 The changes in trend of (a) conductivity labelled A–F and (b) pH labelled A–F during the course of PCC precipitation process.
RESEARCH ARTICLE
O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
4
Figure 3 (a) Sudden and briefly peak-up in resistivity values and (b) exothermic heat released during the recarbonation process at 379.5 mL min–1 gas.
values, which occurred at the same ‘dropped points’ as in pH
and conductivity trends. TDS values were also exhibiting similar
trend as in conductivity and pH.
The TDS (mg L–1) usually drop to zero simultaneously with pH
and conductivity at the same moments. Higher reactant concentration of 2.0 M and 1.5 M (PCC-E2 and PCC-F2) requires longer
recarbonation time. This can also be shortened by increasing the
gas flow rate. However, morphology and other crystal characteristic of resulted PCC would not be the same. PCC precipitation is
an exothermic chemical reaction. In this study, the rise in temperature during the course of precipitation were observed in the
order of 10 to 15 °C (Fig. 3). Higher reactant concentrations (e.g.
2.M in PCC F2) yielded greater amount of heat as compared to
the lower one.
3.2.1. XRD and FTIR Analysis
Figure 4 shows the gradual changes in the XRD patterns of
PCC produced at various operating variables and conditions.
Gradual increase in intensity was in tandem with phase concentration or growth rate of PCC followed by the reduction of
Ca(OH)2. Irrespective of reactant temperature, the growth rate
of PCC increased with longer bubbling times. It seems that operating temperature around 50 °C favoured crystallization of
rhombohedra calcite PCC. XRD pattern for experimental design
as shown in Table 1 is almost identical and consistent because
of the complete formation to calcite phase in all experimented
samples. All the samples show almost similar peak position and
intensity. No peaks of Ca(OH)2 was present, thus indicating a
complete recarbonation. Marked different in crystal morphology
characteristics as shown by SEM micrographs (Fig. 5) can only be
revealed by Fourier transform infrared spectroscopy (FTIR).
FTIR spectrum shows typical absorption peaks of calcite around
1433, 874 and 713 cm–1 (Fig. 4). This indicates that the formed
PCC have, somewhat, variation in crystal morphology or atomic
structure during precipitation, which is hard to be defined by
the XRD spectrum.
3.2.2. Chemical Composition and Purity
Tables 2 and 3 presents the results of XRF and LOI analysis.
LOI is best used to determine CaCO3 content. The mass loss
corresponds to the released amount of CO2 (44 wt.%) during the
firing process above dissociation temperature of limestone.
Figure 4 (a) XRD and (b) FTIR spectra of selected synthesized PCC produced at different operating variables and conditions.
RESEARCH ARTICLE
O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
5
Figure 5 Crystal morphology and particles size distribution of lower (A2, D3) concentration and higher concentration (D2, B2, F2) PCC from this experiments and also PCC-P8 after Ariffin.21
Results indicated that the produced PCC is very high in purity
with an average of about 98.6 %. Other components, which are
normally inherited from precursor stone are often less than
1.5 %.
3.2.3. Crystal Morphology
Figure 4 shows examples of the crystal phases of PCC produced
at the selected concentrations, gas flow rates and bubbling times.
Generally, calcium carbonate mineral phases crystallized into
three common crystal polymorphs (calcite, aragonite and
vaterite). However, there are other crystal morphologies that
have been discovered depending on different kind of operating
techniques and reactant used. At ambient conditions, calcite is a
thermodynamically stable form of carbonate and has cubical
rhombohedra crystal morphology. At a lower reactant concentration (e.g. PCC-A1-0.2 M), well-developed cubic-like, rhombohedra calcite were generally formed within the size order of 50
to 100 nm (Fig. 5). This result obtained by using the modified
reactor can be compared with results by several authors using
chemical additives to achieve smaller PCC particle sizes.
Konopacka-ºyskawa et al.17 synthesized PCC particles using
aqueous of isopropyl alcohol, n-butanol and glycerol as solvent
on lime slurry using carbonation process. They reported that an
increment of reactive mixture of isopropyl alcohol and
n-butanol concentration resulted in the high yield of smaller
PCC particles of approximately 2.5 µm. However, finer PCC
particles size of 0.1–0.59 µm were formed when single solution of
20 % glycerol concentration was used. Feng and Yon18 reported
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O.A. Jimoh, T.A. Otitoju, H. Hussin, K.S. Ariffin and N. Baharun,
S. Afr. J. Chem., 2017, 70, 1–7,
<http://journals.sabinet.co.za/sajchem/>.
Table 3 LOI (%) of synthesized PCC.
Table 2 XRF Analysis of PCC.
Component
Al2O3
SiO2
K2O
CaCO3
Fe2O3
NaO
CuO
SrO
P2O3
MgCO3
6
/wt.%
PCC A2
PCC D1
0.17
0.24
0.015
99.0
0.034
0.019
0.10
–
–
–
0.15
0.19
Trace
98.0
0.014
0.02
0.10
0.02
0.03
1.1
Expt.
–sample no
PCC A1
PCC A2
PCC B1
PCC B2
PCC C1
PCC D1
PCC D2
PCC E1
PCC E2
PCC F2
Before fired
/g
After fired
/g
LOI
/%
CaCO3
/%
1.0017
1.0003
1.0060
1.0015
1.0020
1.0009
1.0002
1.0054
1.0029
1.0008
0.5672
0.5673
0.5641
0.5598
0.5601
0.5620
0.5592
0.5627
0.5685
0.5701
43.38
43.29
43.93
44.10
44.10
43.85
44.09
44.03
43.31
43.04
98.60
98.38
99.84
100.0
100.0
99.66
100.0
100.0
98.43
97.81
Figure 6 Crystallization hypothesis for PCC production in the liquid-gas system from Ca(OH)2 suspension.
an average particle size of 1 to 3 µm at ambient temperature
when EDTA and terpineol were used. Studies by Xiang et al.19
also reported that addition of 1 % (w/w) ZnCl2 shows an obvious
decrease in PCC particle size and 0.2 µm diameter of spherical
PCC particles was formed.
Further increase of flow rate (379 mL min–1) resulted in slightly
coarser PCC (PCC-A2). This is consistent with the work by
Agnihotri et al.20, that the particle size decreases with decrease in
CO2 concentration. Reactant concentration (PCC-B1-0.5M) often
doubled the crystal size of these rhombohedra PCC within the
order of 100 to 150 nm. Higher gas flow rate at 379 mL min–1 does
not result to much change in size of this PCC-B2. At the
224 mL min–1 gas flow rate, the resultant PCC-D1 retained its
initial rhombohedron shape. Gradual transformation of calcite
rhombohedra to prismatic (seed-like) calcite morphology was
only observed at the 1.0 M with 379.5 mL min–1 (PCC-D2) gas
flow rate. This PCC-D2 possesses a crystal dimension of 175 nm
by 100 nm. At the higher concentration of 2.22 M (PCC-P8),
scalenohedron calcite PCC was precipitated. ‘Milk of lime’ concentrations higher than 1.0 M, usually exhibit prismatic
‘seed-like’ PCC morphology (PCC-F1 and PCC-F2). They usually
possess larger particle sizes than 400 nm by 120 nm. The PCC
crystallization or formation in relation to the liquid–gas reaction
such as in this study could be demonstrated by the reaction
diagram (Fig. 6).
A study by Ariffin21 showed that concentration higher than
2.0 M with prolonged bubbling time would only result in
rosette-like scalenohedron PCC formation.
4. Conclusions
‘Milk of lime’ concentration, gas flow rate and residence time
are the main operating parameters and variables in PCC produc-
tion control. By varying such variables and conditions, we were
able to understand and manipulate crystal morphology and
other characteristics like crystallite size and distribution patterns
by using limestone as a starting commercial material. A low reactant concentration (0.2 M) favours the formation of cubic-like,
rhombohedra calcite with almost uniform particle size distribution. Increasing the reactant concentration to specific levels, e.g.
up to 0.56 M, prismatic calcite morphology start to emerge. At
higher level ‘milk of lime’ concentration such as at 2.0 M and
2.2 M, scalenohedron calcite was gradually formed. The crystallization hypothesis would be useful in predicting the production
of desired PCC characteristics rather than varying those operating variables. This model diagram can easily be modified to
include effects of reactant temperature, pressure and presence
of additives later.
Acknowledgement
The second author acknowledges Universiti Sains Malaysia
(USM) for an e-science fund fellowship grant.
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